Natural environmental change plays an important role in structuring diversity across space and time. However, anthropogenic impacts are increasing at unprecedented rates, disrupting natural fluctuation regimes, introducing novel disturbances, impairing ecological processes and threatening biodiversity. Hence, understanding whether fluctuations in environmental conditions generate different communities is of substantial importance, as communities might respond differently to future disturbance events.
Here we explore how the presence of environmental fluctuation influences assembly and diversity in metacommunities. We use a microcosm experiment where metacommunities were composed of five equally connected patches, having the same initial species assemblage of nine protozoan species. Regional resource availability was high, but heterogeneous across each metacommunity. Environmental fluctuations were implemented by manipulating light availability between light and dark. We compared the control of constant light, to a treatment of “low” frequency of fluctuations, and a treatment of “high” frequency of fluctuations. Fluctuation cycles occurred across species’ generation time for a total of ~60-280 generations, depending on species’ growth rate.
At the patch/community level, increased environmental fluctuation had a positive effect on local productivity. Alpha diversity was negatively impacted, as a result of an increase in local richness and loss of evenness. At the metacommunity level, increased environmental fluctuation also had a positive effect on local productivity. Gamma diversity showed an inverted hump-shaped curve, resulting from a similar pattern for richness and a decline in evenness. Dissimilarity in productivity between patches and metacommunity responses increased with environmental fluctuation. Beta diversity showed an inverted hump-shaped curve, due to a similar pattern for evenness, whereas an effect of richness was undistinguishable.
Overall, the presence of environmental fluctuation increased productivity, yet, its effects on diversity were dependent on the frequency of fluctuations. The “low” frequency treatment showed lower beta diversity in comparison to the other treatments, with lower richness and evenness at both patch and metacommunity levels. The “high” frequency treatment showed beta diversity values comparable to the constant environment treatment, however, due to higher richness and lower evenness at both patch and metacommunity levels. This suggest that environmental fluctuation is having a differential impact among species, directly affecting abundance and competitive interactions.
We demonstrate that in a heterogeneous metacommunity, where resource and connectivity are not limiting, environmental fluctuation can lead to the assembly of different communities in respect to diversity properties. How this scales to define metacommunities responses to future disturbances events is subject to a following study.